Three different projects using similar methods are in various stages of completion. The first involved determining the 3-D structure of the Drosophila genome by mapping all chromosome arms in differentiated nuclei. A series of unique DNA sequences cloned in E. coli are being used besides 5S RNA and 18 plus 28S RNA. Several highly repeated sequences, as cloned DNA, have recently become available serving as additional probes. They are useful because the chromocenter and the chromosome ends are marked. The in situ methods have improved to the point to where unique sequences can be mapped on human chromosomes. Several new sequences, the snRNAs, are being mapped in Drosophila to be followed with human chromosomes.